Sensor circuit

ABSTRACT

A method of measuring signals related to a photodiode based sensor and calculating a corrected data value thereof is disclosed. A nominal reset voltage value of the photodiode may be measured. A knee point voltage may be applied to the photodiode and resets a voltage on the photodiode to the knee point voltage when the voltage on the photodiode falls below the knee point voltage. Applying the knee point voltage may extend the dynamic range of the sensor. An output voltage of the photodiode at end of an integration time of the photodiode may be measured. The knee point voltage may be applied again after the end of the integration time. A voltage value of the photodiode of the knee point voltage may be measured. The nominal reset voltage value, the output voltage of a sensor and the knee point voltage may be reported to calculate the corrected data value.

TECHNICAL FIELD

Embodiments of the present invention relate to the field of electronics.More particularly, embodiments of the present invention relate toextending a dynamic range of a sensor circuit.

BACKGROUND

In general, light sensor elements, or pixels, can be subjected todifferent brightness and light intensity. Each pixel reaches asaturation level at a different time based on the detected brightness.In other words, the slope of each pixel will vary depending on theamount of light reaching the pixel, where the slope is its voltageplotted over time, e.g., the voltage as the y-axis and the time as thex-axis.

Accordingly, a pixel with a greater slope reaches saturation levelfaster in comparison to other pixels. When the saturation level duringthe integration time of a pixel is reached the collection of brightnessinformation for that pixel stops.

SUMMARY

In one embodiment, an offset voltage introduced when a pixel voltage isreset to a given threshold voltage is measured in order to extend thepixel dynamic range during integration time. The measured offset voltagemay be corrected in order to maximize image quality. The offset voltagemay be measured and corrected with minimal impact on the amount ofmemory and processing power specified, e.g., by linearizing the pixelbehavior as discussed in more detail herein,

In one embodiment of the present invention, a nominal reset voltagevalue of a photodiode (e.g., a pixel sensor) may be measured. Forexample, the nominal reset voltage value may be measured by resetting acollection capacitor and measuring its change. At a set time over theintegration time of the pixel, a voltage value of the photodiode may bereset to a first knee point voltage value when the voltage value of thephotodiode lies below the first knee point voltage value. At the end ofthe integration time of the pixel, the voltage value of the photodiodemay be transferred to the collection capacitor by transferring a chargeassociated therewith. Measuring the collected charge on the collectioncapacitor provides the voltage measurement for the pixel at the end ofthe integration time.

Because the first knee point voltage value may vary from one pixel tothe next, the first knee point voltage value may be measured in order tocompensate for the offset value introduced, and resulting from resettingthe photodiode to the first knee point voltage. Accordingly, thephotodiode may be initialized to a predetermined value, e.g., reset toits lowest voltage value. The collection capacitor may be reset and asecond knee point voltage signal may be subsequently asserted for thesame pixel. The second knee point voltage signal may be approximatelythe same amplitude as the first knee point voltage value. The chargeassociated with the second knee point voltage signal may be collected onthe capacitor and subsequently measured, as discussed above. Measuringthe charge collected on the capacitor provides the second knee pointvoltage value, which may be approximately the same as the first kneepoint voltage value. Therefore, the offset introduced to a given pixelas a result of the knee voltage reset may be directly measured andrecorded for each pixel sensor circuit.

According to one embodiment, the corrected data value of a pixel sensormay be the nominal reset voltage value minus the output pixel voltage atthe end of the integration time, when the nominal reset voltage valueminus the output pixel voltage all divided by a gain may be greater thanthe nominal reset voltage value minus the first knee point voltage. Thecorrected data value may be the first knee point voltage value minus theoutput pixel voltage at the end of the integration time all multipliedby a gain plus the nominal reset voltage value minus the first kneepoint voltage value when the nominal reset voltage value minus theoutput pixel voltage all divided by the gain at the end of integrationtime may be less than the nominal reset voltage value minus the firstknee point voltage. Accordingly, the corrected data may be compensatedvalue for the offsets introduced by the pixel sensor, thereby maximizingthe image quality. Moreover, the offset values introduced may becalculated with minimal impact on the amount of circuitry used for thepixel sensor and the response of the pixel sensor becomes linearized.

An embodiment pertains to a method of measuring signals related to aphotodiode having a wide dynamic range. The method includes measuring anominal reset voltage value of the photodiode. A first knee pointvoltage may be asserted for the photodiode. In one embodiment, the firstknee point voltage extends a dynamic range of the photodiode byresetting a voltage on the photodiode to the first knee point voltagefor any photodiode voltage below the first knee point voltage. The firstknee point voltage may be asserted at a predetermined time after a startof the integration time and prior to an end of the integration time. Anoutput voltage associated with the photodiode at the end of anintegration time of the photodiode may be measured.

Subsequent to the end of the integration time, a second knee pointvoltage may be asserted for the photodiode. The second knee pointvoltage may have a value approximately equal to a value of the firstknee point voltage. Subsequent to asserting the second knee pointvoltage, a voltage value of the photodiode corresponding to the secondknee point voltage may be measured. The nominal reset voltage value, theoutput voltage and the voltage value of the photodiode after the secondknee point voltage may be applied may be reported, e.g., stored.

Measuring the nominal reset voltage value may include resetting acapacitor. A first charge (the nominal voltage) may be collected andmeasured on the capacitor prior to transferring a second charge (i.e.,the data value) from the photodiode to the capacitor.

According to an embodiment, the data value the output voltage may bemeasured by transferring the charge from the photodiode to the capacitorat the end of the integration time. The charge may be collected by thecapacitor. Accordingly, the collected charge may be measured where thecollected charge may be related to the voltage associated with thephotodiode at the end of the integration time and as effected by thefirst knee point voltage applied thereto.

According to an embodiment, the voltage value of the photodiodecorresponding to the second knee point voltage may be measured byinitializing the photodiode to a predetermined value, e.g., lowestvoltage value for the photodiode, e.g., after the integration time ends.It is appreciated that the capacitor may be reset and the photodiode maybe initialized prior to asserting the second knee point voltage. Acharge associated with the second knee point voltage may be collected bythe capacitor. Accordingly, the collected charge on the capacitor may bedirectly measured.

The measured values, e.g., the nominal reset voltage value, the secondknee point voltage value and the output voltage value, may be used tocalculate the corrected data value. According to one embodiment, thecorrected data value may be the nominal reset voltage minus the outputvoltage value when the nominal reset voltage value minus said outputvoltage value all divided by a gain may be greater than the nominalreset voltage value minus the voltage value of said photodiode. In oneembodiment, the corrected data value may be the voltage value of thephotodiode minus the output voltage value all of which are multiplied bya gain plus the nominal voltage value minus the voltage value of thephotodiode when the nominal reset voltage value minus the output voltagevalue all divided by the gain may be less than the nominal reset voltagevalue minus the voltage value of the photodiode. According to anembodiment, the gain may be a duration of the integration time dividedby an elapsed time measured at the start of when the first knee pointvoltage may be applied until the end of integration time.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are illustrated by way of example,in the figures of the accompanying drawings and in which like referencenumerals refer to similar elements and in which:

FIG. 1 shows behavior of pixel sensor circuits upon application of aknee point voltage to extend the dynamic range of pixels in accordancewith an embodiment of the present invention.

FIGS. 2A and 2B show circuit diagram and a timing diagram, respectively,in accordance with an embodiment of the present invention.

FIG. 3 shows a block diagram for extending a dynamic range of a pixel naccordance with an embodiment of the present invention.

FIG. 4 shows pixels and circuits in accordance with an embodiment of thepresent invention.

FIG. 5 shows flow diagram for measuring values used in calculating acorrected data value in accordance with an embodiment of the presentinvention.

FIG. 6 shows flow diagram for calculating an offset value in accordancewith one embodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. Embodiments are intended to cover alternatives, modificationsand equivalents, which may be included within the spirit and scope ofthe disclosure.

Referring now to FIG. 1, behaviors of pixel sensor circuits, e.g.,pixels, upon application of a knee point voltage to extend the dynamicrange of pixels in accordance with one embodiment of the presentinvention are shown. The pixels may be located within an array of pixelsof an image sensor. The voltage responses of four pixels P1, P2, P3 andP4 are shown. It is appreciated that each pixel may be associated with adifferent photodiode. Each pixel is exposed to a different brightnessand therefore reaches saturation level at a different time. For example,the first pixel P1 and the second pixel P2 never reach saturation whilepixels P3 and P4 reach saturation at different times. In other words, asteeper slope indicates that saturation will be reached faster.

In this embodiment, pixels P3 and P4 reach saturation level during theintegration time and may result in less than optimal image quality.Application of a knee point voltage, e.g., C, to the photodiodesassociated with pixels P3 and P4 prevent pixels P3 and P4 from reachingsaturation level during integration time. In other words, the knee pointvoltage, e.g., C, resets the voltage associated with the photodiodes ofpixels P3 and P4 to the knee point voltage value at time t₁ because thevoltage values of P3 and P4 are less than the knee point voltage value,e.g., C. As a result, the dynamic range of pixels, e.g., P3 and P4, thatotherwise would have reached saturation level are extended. Resettingphotodiodes associated with pixels P3 and P4 at time t₁ results in pixelvoltages 63 and 64 respectively at the end of integration time. It isappreciated that voltages associated with pixels P3 and P4 are bothgreater than the saturation level at the end of the integration time.

It is appreciated that the knee point voltage C may be asserted at apredetermined time, e.g., t₁, which occurs prior to end of integrationtime. Application of a knee point voltage to photodiodes associated withpixels P1 and P2 does not reset their respective voltages at time t₁because the voltage values of pixels P1 and P2 are higher than the kneepoint voltage value C at time t₁. At the end of integration time, pixelsP1 and P2 have voltage values B1 and 62 respectively, both of which areabove the saturation level.

As a result of asserting the knee point voltage, pixels P3 and P4 havevoltage values 63 and 64 respectively at the end of integration timeinstead of reaching saturation level. In other words, application of theknee point voltage at a predetermined time during the integration timeresets a photodiode associated with a pixel if the pixel voltage isbelow the knee point voltage value, e.g., C, at t₁.

It is appreciated that the pixel sensor circuitry may have some residualcharge associated therein. For example, despite a complete reset, somecharge may remain on the pixel capacitance, resulting in a voltagereferred to as the nominal reset voltage, e.g. A. Compensating for thisnominal reset voltage maximizes image quality and pixel linearity ofresponse.

It is appreciated that the knee point voltage may reset the photodiodesmultiple times during the integration time according to one measurementtechnique. As such, resetting the photodiodes associated with pixels P3and P4 once is illustrative and not intended to limit the scope of thepresent invention.

As presented above, application of the knee point voltage C introducesan offset voltage that can be measured and corrected for in order tomaximize image quality, In order to calculate the corrected datavoltage, the knee point voltage value, e.g., C, a nominal reset voltage,e.g., A, and the pixel voltage value at the end of integration time,e.g., B1, B2, B3 and B4 are measured in accordance with embodiments ofthe present invention. It is appreciated that the measurement of thesevalues and subsequent calculation of the corrected data voltage occurswith minimal impact on the amount of memory circuitry used by the pixelsensor.

Referring now to FIG. 2A, a circuit diagram 200A of a pixel sensorcircuit in accordance with one embodiment of the present invention isshown. The circuit 200A comprises an initialization switch 210, aphotodiode 240, a transfer switch 220, a reset switch 230, a collectioncapacitor 250 and a readout switch 260. A voltage line 280 is V_(dd).

According to one embodiment of the present invention, the initializationswitch 210 may be used to initialize the photodiode 240. For example,the initialization switch 210 may be an anti-blooming transistor thatinitializes the photodiode 240 to a high voltage.

Moreover, it is appreciated that the initialization switch 210 may beused to apply a knee point voltage at its gate. For example, the outputvoltage of the photodiode 240 may reset to the knee point voltage value,e.g., C, at a predetermined time if the output voltage of the photodiode240 may lower than the knee point voltage, e.g., C, assuming C may beasserted at the gate of 210. On the other hand, the output voltage ofthe photodiode 240 may be unaltered at a predetermined time if theoutput voltage of the photodiode 240 may be greater than the knee pointvoltage, e.g., C, as applied at the gate of the initialization switch210.

The transfer switch 220 may be used to transfer a charge from the outputof the photodiode 240 to the capacitor 250. The reset switch 230 may beused to reset the capacitor 250. The readout switch 260 may be used tomeasure a charge collected on the capacitor 250.

Referring now to FIG. 2B, a timing diagram 200B in accordance with oneembodiment of the present invention is shown. Generation of the timingdiagram 200B enables measurement of the nominal reset voltage, e.g., A,the knee point voltage, e.g., C, and the pixel voltage at the end of theintegration time, e.g., B1, B2, B3 and B4. It is appreciated thatwaveforms 280′, 230′, 220′, 210 and 260′ control their respectivecomponents 280, 230, 220, 210 and 260.

According to one embodiment, the photodiode 240 may fully reset (to highvoltage) and initialized by asserting a signal 210 at time t₀. Resettingthe photodiode 240 at time t₀ starts the integration time, e.g.,T_(int), of a pixel. The capacitor 250 may reset by asserting signal 230at time t₂. It is appreciated that the transfer switch 220 may beasserted in order to transfer a charge from the photodiode 240 to thecapacitor 250. Accordingly, charge collected by the capacitor 250 afterhaving just been reset may be the charge associated with the nominalreset voltage, e.g., A. In order to measure the charge collected by thecapacitor 250, the readout signal 260′ may be asserted at time t₃.Accordingly, the nominal reset voltage, e.g., A, may be measured anddetermined according to the above,

While the above measurement of A was being done, the photodiode 240 wasresponding to the light on the pixel. It is appreciated that the kneepoint voltage may be applied at t₁. The knee point voltage may beasserted during the integration time of the pixel. As presented above,application of the knee point voltage causes the output of thephotodiode 240 to reset to the knee point voltage, e.g., C, if theoutput voltage of the photodiode 240 may be less than the knee pointvoltage at t₁. Otherwise, the output voltage of the photodiode 240remains the same without resetting to the knee point voltage.

It is appreciated that in this embodiment, the knee point voltage may beasserted when the pixel may be 80-90% into its integration time periodbut this can vary. It is also appreciated that the knee point voltagemay be applied more than once. It is further appreciated that resettingthe capacitor 250 during integration time at time t₂ and reading itsvalue at t₃ may be either before or after the application of the kneepoint voltage at time t₁.

Asserting signal 220′ at time t₄, which is at the end of the integrationtime period, transfers the charge associated with the output of thephotodiode 240 to the capacitor 250. Accordingly, the capacitor 250collects a charge associated with the output of the photodiode 240. Inorder to measure the collected charge on the capacitor 250, the readoutsignal 260′ may be asserted at time t₅. The readout value at time t₅ isassociated with the voltage value of the pixel, e.g., B1, B2, B3 and B4,at the end of the integration time.

As presented above, the knee point voltage, e.g., C, may be unknown forthe given pixel and varies from one pixel to the next. In order tomeasure the knee point voltage value, e.g., C, the photodiode 240 may beinitialized to a predetermined value, e.g., lowest voltage, at time t₆by de-asserting the signal 280′ and pulsing signals 230′ and 220′ attime t₆.

The value of the photodiode 240 may be known when it is initialized to apredetermined value, e.g., lowest voltage. Asserting the knee pointvoltage that may be unknown and measuring the collected charge resultingfrom initialization of the photodiode 240 and the application of theknee point voltage provides a measurement for C. Accordingly, afterinitializing the floating diffusion and photodiode 240 to a low voltage,the knee point voltage 210′ may be asserted once again at time t₇. It isappreciated that the knee point voltage applied may be approximately thesame as the knee point voltage applied at time t₁.

The capacitor 250 may reset by asserting the signal 230′ at time t₈. Itis appreciated that the capacitor 250 may be reset any time after theinitialization of the photodiode 240 but before transferring a chargeassociated with the knee point voltage asserted at time t₇ to thecapacitor 250 After the capacitor 250 may reset, the charge associatedwith the assertion of the knee point voltage at time t₇ may betransferred, e.g., by asserting the 220′ signal at t₉, from the outputof the photodiode 240 to the capacitor 250. The transferred charge maybe collected by the capacitor 250. The collected charge may beassociated with the assertion of the knee point voltage at time t₇.Accordingly, measuring the collected charge on the capacitor 250provides a value associated with the knee point voltage. As such, thereadout signal 260′ may be asserted at time t₁₀ n order to measure thecollected charge on the capacitor 250.

Accordingly, the values of the nominal voltage reset, e.g., A, the pixelvoltage at end of the integration time, e.g., B1, B2, B3 and B4, and theknee point voltage value, e.g., C, are all measured using the timingscheme discussed above. The measured values may be reported and storedfor the given pixel. For example, the measured values may be stored in amemory component. The measured values may be used to calculate thecorrected data value for the pixel. This is done for each pixel in thepixel array.

According to one embodiment of the present invention the corrected datavalue may be:

Corrected data value=(A−B) if (A−B)/gain>(A−C),

where A may be the measured nominal reset voltage value, B may be thepixel voltage at the end of the integration time, C may be the measuredknee point voltage value and gain may be the total integration timedivided by the first slop integration time. In other words, thephotodiode 240 associated with the pixel has not reset to the knee pointvoltage during the integration time if the nominal reset voltage valueminus the pixel voltage at the end of the integration time all dividedby a gain may be greater than the nominal reset voltage value minus theknee point voltage value. Subtracting the measured nominal reset voltagefrom the pixel voltage at the end of the first slope integration timecompensates for charge in the system that may be unrelated to the pixelsresponse to the light, e.g., noise. Thus, accounting for the nominalreset voltage maximizes the image quality and linearity of the pixelresponse.

The corrected data value may be:

Corrected data value=[(A−C)+gain*(C−B)] if (A−B)/gain<(A−C),

where A may be the measured nominal reset voltage value, B may be thepixel voltage at the end of the integration time and C may be themeasured knee point voltage value. The gain may be associated with theration of the integration period and the time which the knee pointvoltage may be asserted in order to linearize the behavior of the pixel.According to one embodiment, the gain may be:

gain=T _(int)/Knee point time.

In other words, the photodiode 240 associated with the pixel has beenreset to the knee point voltage during the integration time because thenominal reset voltage value minus the pixel voltage at the end of theintegration time all divided by the gain may be lower than the nominalreset voltage value minus the knee point voltage. Accounting for thenominal reset voltage A maximizes image quality, as discussed above. Thecorrected data value presented above, linearizes the pixel behavior bytaking into consideration the photodiode behavior before and afterapplication of the knee point voltage. In other words, the discontinuityand non-linearization introduced as a result of applying the knee pointvoltage C may be eliminated.

Referring now to FIG. 3, a block diagram 300 for extending a dynamicrange of a pixel in accordance with one embodiment of the presentinvention is shown. The block diagram 300 comprises a signal generationcircuit 320, a pixel sensor circuit 310 and a data read and correctioncircuit 330. The signal generation circuit 320 generates control signalsas shown in waveforms 200B, as presented above. The circuit 310 issubstantially similar to the circuit 200A, as presented above. Thecircuit 310 reports the measured values, e.g., nominal reset voltage A,the pixel voltage at the end of integration time, B1, B2, B3 and B4 andthe value of the knee point voltage to the data read and correctioncircuit 330. It is appreciated that the reported measured values may bestored in a memory component 340 within circuit 330. The data read andcorrection circuit 330 may use the measured values to calculate thecorrected data values, as presented above.

FIG. 4 shows an array of pixels and sensor circuits 400 in accordancewith one embodiment of the present invention. The pixels and circuits400 are arranged in an array of m rows by n columns as might be found ina digital capture device. It is appreciated that each pixel may have arepresentative sensor circuit 200A associated therewith, as presentedabove.

FIG. 5 shows a flow diagram 500 for measuring values used in calculatinga corrected data value in accordance with one embodiment of the presentinvention. At step 510, the nominal reset voltage value of thephotodiode 240, e.g., C, may be measured. For example, the nominal resetvoltage value may be measured by resetting the capacitor 250 at time t₂(FIG. 2B) and by collecting and measuring the charge on the capacitor250 at time t₃.

At step 520, a first knee point voltage may be applied at apredetermined time, e.g., t₁, after the start of the integration period.It is appreciated that the photodiode 240 may reset at the start of theintegration period. The application of the knee point extends a dynamicrange of the photodiode by resetting a voltage on the photodiode to theknee point voltage if the voltage on the photodiode is below the kneepoint voltage value at t₁. For example, the voltage values of pixels P3and P4 (FIG. 1) are below the knee point voltage C at time t₁. Thus, thevoltage value of the photodiode may reset to the knee point voltage forpixels P3 and P4 at t₁ but remain unchanged for pixels P1 and P2 sincetheir value may be greater than the knee point voltage, e.g., C, t₁.

At step 530, the output voltage associated with the photodiode, e.g.,pixels P1, P2, P3 and P4, at the end of the integration time may bemeasured. For example, the output voltage for pixels P1, P2, P3 and P4are measured by asserting the transfer switch 220 (of each sensor) totransfer a charge from the photodiode 240 to the capacitor 250 at timet₄. The charge collected on the capacitor 250 may be measured byasserting the readout signal 260 at time t₅.

At step 540, the photodiode 240 may be initialized to a predeterminedvalue, e.g., low voltage, at time t₆ (FIG. 2B). At step 550, a secondknee point voltage may be asserted at the photodiode 240 at time t₇. Itis appreciated that the second knee point voltage has approximately thesame voltage value as the first knee point voltage. At step 560, avoltage value of the photodiode 240 corresponding to the second kneepoint voltage may be measured. For example, the capacitor 250 may resetby asserting the reset signal 230′ at time t₈ (FIG. 2B). It isappreciated that the capacitor 250 may reset after the end of theinitialization of the photodiode, e.g., t₆, but prior to the transfer ofthe charge, e.g., t₉, associated with the second knee point from thephotodiode 240 to the capacitor 250.

Asserting the transfer signal 220′ at time t₉ transfers the charge fromthe photodiode 240 to the capacitor 250. The transferred charge may beassociated with the application of the second knee point voltage. Thus,the capacitor 250 collects a charge associated with the second kneepoint voltage. Asserting the readout signal 260′ at time t₁₀ measuresthe collected charge on the capacitor 250. As a result, the value of thesecond knee point voltage may be measured which may be approximately thesame as the first knee point voltage with respect to the pixel circuit.

At step 570, the measured values, e.g., nominal reset voltage A, theknee point voltage C and the pixel voltage at the end of the integrationtime B may be reported to the data read and correction circuit 330. Atstep 580, the measured values may be stored in memory for subsequentcorrection.

FIG. 6 shows a flow diagram 600 for calculating a corrected data valuein accordance with one embodiment of the present invention. At steps610, 620 and 630, the reported values, as discussed above are accessedin no particular order. At step 640, the reported values may be used tocalculate the corrected data value, as presented above. For example, thecorrected data values are computed by:

Corrected data value=(A−B) if (A−B)/gain>(A−C), and

Corrected data value=[(A−C)+gain*(C−B)] if (A−B)/gain<(A−C).

At step 650, the corrected data value may be stored. Correcting thepixel value linearizes the pixel behavior. it is appreciated that themeasurements and corrections are performed with minimal impact on memoryusage and sensor circuitry.

It is appreciated that embodiments described herein may be extended tomultiple knee voltages asserted at different times during theintegration period. Multiple knee voltages result in an increase in thenumber of stored knee voltage references and may need additionalcorrection terms for each additional knee voltage point asserted.

Furthermore, it is appreciated that although specific circuitry isillustrated, other configurations may be equally used. For example, inone embodiment a knee point voltage may be applied to the storage nodecapacitor 250 through the reset switch 230 while continuouslyintegrating the photocurrent induced in photodiode 240. In anotherembodiment, a knee point voltage may be applied to the storage nodecapacitor 250 through the reset switch 230 while continuously turning onthe transfer switch 220 to connect the photodiode 240 and the capacitor250 or by periodically turning the transfer switch 220 to periodicallytransfer charge from the photodiode 240 to the capacitor 250.

In the foregoing specification, embodiments of the invention have beendescribed with reference to numerous specific details that may vary fromimplementation to implementation. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

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 22. A method for adjusting a pixel voltage,comprising: applying a first voltage to a photodiode after anintegration period, the first voltage sufficient to substantially fillthe photodiode with electrons; and applying a second voltage to thephotodiode after applying the first voltage and after the integrationperiod; reading a voltage of the photodiode after applying the secondvoltage.